In recent years, scanning microscopes based on optical, electrical, or atomic force sensing have proven to be useful in a variety of disciplines. For example, the use of such microscopes is a useful adjunct to scanning electron microscopy (SEM) because contrast mechanisms alternative to those of SEM may be employed to explore the topography and composition of surfaces.
Atomic force microscopes have been described, in which a sharply pointed probe tip is brought near the surface of a sample, and forces acting normal to the surface affect the motion of the tip. For example, U.S. Pat. No. Re. 33,387, issued to G.K. Bining on Oct. 16, 1990, describes such an arrangement where the tip is mounted on a cantilever beam which is deflected as a result of the normal forces. A further example is U.S. Pat. No. 4,851,671, issued to D. W. Pohl on Jul. 25,1989, which describe an alternative arrangement in which the tip is mounted on an oscillating crystal, and the frequency of oscillation is altered by the presence of normal forces.
When there is relative, lateral motion between the probe tip and the sample surface, forces are also present which act parallel, rather than normal, to the surface. (By "lateral" is meant that the relative displacement has a component which is, on the average, parallel to the sample surface in the vicinity of the probe). Such shear forces may arise, for example, from static or dynamic friction between the tip and the sample. Other shear forces arise as viscous forces in the air or other fluid medium between the tip and the sample.
A probe which senses shear forces can offer some advantages over conventional, normal-force-sensing probes. For example, shear forces generally act over greater distances, measured from the sample surface, than do normal forces. The farther from the surface a probe can act, the smaller is the likelihood of damaging the probe or the sample. Acting at a greater distance also makes it possible to scan more quickly, because collisions with high surface features are more easily avoided. Furthermore, a cantilever-mounted normal-force probe generally needs to be rather stiff along the lateral direction. By contrast, a shear-force probe can be more flexible along that direction, reducing the likelihood of damage by lateral collisions with the wall of surface features on the sample.
A probe which, in a limited context, senses forces which are directed parallel to a sample surface has been described in U.S. Pat. No. 4,992,659 issued to D. W. Abraham, et al., on Feb. 12, 1991. That patent describes a scanning, tunneling microscope which has a probe adapted to detect magnetic field in the sample being scanned. Magnetic fields in the sample act on the tunnel current in the probe to cause a Lorentz-force deflection of the probe parallel to the sample surface. Such a probe is limited in use to metallic samples having magnetic properties, and is subject to the same dangers of probe or sample damage as are conventional, normal-force-sensing probes.
Despite its advantages, practitioners in the field have hitherto failed to provide a general-purpose-shear-force-sensing probe.